Algorithm incorporating driving conditions into LNT regeneration scheduling

ABSTRACT

A lean NO x  trap is a diesel exhaust aftertreatment system is selectively denitrated based on a measure relating to the state and/or the performance of the exhaust aftertreatment system, or a portion thereof comprising the lean NO x  trap, reaching a critical value. The critical value is varied according to the demands currently being place on the exhaust aftertreatment system. In one embodiment, the critical value is set based on engine speed-load information. The method regenerates more frequently when exhaust aftertreatment demands are high and less frequently when demands are low. The method improves aftertreatment performance while reducing aftertreatment fuel penalty.

FIELD OF THE INVENTION

The present invention relates to diesel power generation systems withexhaust aftertreatment.

BACKGROUND

NO_(x) emissions from diesel engines are an environmental problem.Several countries, including the United States, have long hadregulations pending that will limit NO_(x) emissions from trucks andother diesel-powered vehicles. Manufacturers and researchers have putconsiderable effort toward meeting those regulations.

In gasoline powered vehicles that use stoichiometric fuel-air mixtures,three-way catalysts have been shown to control NO_(x) emissions. Indiesel-powered vehicles, which use compression ignition, the exhaust isgenerally too oxygen-rich for three-way catalysts to be effective.

Several solutions have been proposed for controlling NO_(x) emissionsfrom diesel-powered vehicles. One set of approaches focuses on theengine. Techniques such as exhaust gas recirculation and partiallyhomogenizing fuel-air mixtures are helpful, but these techniques alonewill not eliminate NO_(x) emissions. Another set of approaches removeNO_(x) from the vehicle exhaust. These include the use of lean-burnNO_(X) catalysts, selective catalytic reduction (SCR) catalysts, andlean NO_(x) traps (LNTs).

Lean-burn NO_(X) catalysts promote the reduction of NO_(x) underoxygen-rich conditions. Reduction of NO_(X) in an oxidizing atmosphereis difficult. It has proven challenging to find a lean-burn NO_(x)catalyst that has the required activity, durability, and operatingtemperature range. A reductant such as diesel fuel must be steadilysupplied to the exhaust for lean NO_(x) reduction, introducing a fueleconomy penalty of 3% or more. Currently, peak NO_(x) conversionefficiencies for lean-burn NO_(x) catalysts are unacceptably low.

SCR generally refers to selective catalytic reduction of NO_(x) byammonia. The reaction takes place even in an oxidizing environment. TheNO_(x) can be temporarily stored in an adsorbent or ammonia can be fedcontinuously into the exhaust. SCR can achieve high levels of NO_(x)reduction, but there is a disadvantage in the lack of infrastructure fordistributing ammonia or a suitable precursor. Another concern relates tothe possible release of ammonia into the environment.

LNTs are devices that adsorb NO_(x) under lean conditions and reduce andrelease the adsorbed NO_(x) under rich conditions. An LNT generallyincludes a NO_(x) adsorbent and a catalyst. The adsorbent is typicallyan alkali or alkaline earth compound, such as BaCO₃ and the catalyst istypically a combination of precious metals including Pt and Rh. In leanexhaust, the catalyst speeds oxidizing reactions that lead to NO_(x)adsorption. In a reducing environment, the catalyst activates reactionsby which hydrocarbon reductants are converted to more active species,the water-gas shift reaction, which produces more active hydrogen fromless active CO, and reactions by which adsorbed NO_(X) is reduced anddesorbed. In a typical operating protocol, a reducing environment willbe created within the exhaust from time-to-time to regenerate(denitrate) the LNT.

Regeneration to remove accumulated NOx may be referred to as denitrationin order to distinguish desulfation, which is carried out much lessfrequently. The reducing environment for denitration can be created inseveral ways. One approach uses the engine to create a richexhaust-reductant mixture. For example, the engine can inject extra fuelinto the exhaust within one or more cylinders prior to expelling theexhaust. A reducing environment can also be created by injecting areductant into lean exhaust downstream from the engine. In either case,a portion of the reductant is generally expended to consume excessoxygen in the exhaust. To lessen the amount of excess oxygen and reducethe amount of reductant expended consuming excess oxygen, the engine maybe throttled, although such throttling may have an adverse effect on theperformance of some engines.

Reductant can consume excess oxygen by either combustion or reformingreactions. Typically, the reactions take place upstream of the LNT overan oxidation catalyst or in a fuel reformer. The reductant can also beoxidized directly in the LNT, but this tends to result in faster thermalaging. U.S. Pat. Pub. No. 2003/0101713 describes an exhaust system witha fuel reformer placed in an exhaust line upstream from an LNT. Thereformer includes both oxidation and reforming catalysts. The reformerboth removes excess oxygen and converts the diesel fuel reductant intomore reactive reformate.

Regardless of how the reducing environment is created, it is importantto control the frequency with which reducing conditions are created. Ifthe frequency of regeneration is too low, the LNT will fail to performits function effectively. If the frequency of regeneration is too high,the fuel penalty becomes excessive. In any event, the fuel penalty forregenerating an LNT is a significant factor contributing to theoperating cost of a vehicle using an LNT and it is desirable to keepthat fuel penalty as low as possible while still meeting emissioncontrol objectives.

Numerous methods for scheduling LNT denitrations have been proposed. Thesimplest method is periodic regeneration: regeneration is conductedafter a fixed period of lean operation. This method is generallyimpractical in that NO_(x) accumulation rates vary widely over thecourse of vehicle operation. Using periodic regeneration, either thefuel penalty will be unacceptably high or the emissions control will beunacceptably low.

A prevalent method for scheduling LNT denitration is to schedule basedon LNT loading. LNT loading can be characterized in terms of amount ofNO_(x) accumulated, remaining NO_(x) storage capacity, percentsaturation, or another parameter of this type. Numerous methods forestimating NO_(x) loading and/or remaining NO_(x) storage capacity havebeen proposed. These methods generally involve integrating an estimateof the NO_(x) storage rate and comparing the result to an estimatedNO_(x) storage capacity.

NO_(x) storage rates can be estimated from differences between NO_(x)flow rates out of the engine and NO_(x) flow rates out of the LNT or bymultiplying NO_(x) flow rates out of the engine by estimates of LNTstorage efficiency. Engine out NO_(x) flow rates can be estimatedexclusively from engine operating maps or using a NO_(x) sensor in theexhaust upstream from the LNT. NO_(x) flow rates out of the LNT, whenused, are generally estimated using NO_(x) concentration sensors.

Regenerating based on LNT loading is better than regeneratingperiodically, but is still inaccurate in the sense of resulting inoverly frequent or infrequent regenerations. Aside from any inaccuraciesin measuring NO_(x) storage rates, it is difficult to accuratelydetermine NO_(x) storage capacity. NO_(x) storage capacity varies overtime due to factors including, without limitation, sulfur poisoning,catalyst aging, and catalyst temperature. The degree of saturation atwhich LNT efficiency becomes unacceptably low is also variable being afunction of these and other factors.

Another limitation to regenerating based on NO_(x) loading is that itdoes not take into account the performance of the entire exhausttreatment system. It is known that an LNT can produce ammonia duringdenitration and from this knowledge it has been proposed to combine anLNT and an ammonia SCR catalyst into one system. Ammonia produced by theLNT during regeneration is captured by the SCR catalyst for subsequentuse in reducing NO_(x), thereby improving conversion efficiency over astand-alone LNT with little or no increase in fuel penalty or preciousmetal usage. Regeneration of an LNT in a hybrid system based on LNTloading only may be premature due to performance of the SCR catalyst inaddition to the other factors mentioned above.

An alternative approach is to schedule LNT regeneration based on currentperformance of the aftertreatment system as determined from NO_(x)concentration measurements taken downstream from the LNT. Thesemeasurements can be used on a standalone basis, regenerating when thedownstream concentration exceeds a critical value, or in combination ofwith estimates of NO_(x) concentration upstream from the LNT, wherebythe LNT performance efficiency can be determined and used as a criteria.The performance of the LNT can be determined individually, or theperformance of the LNT in combination with another device, such as anSCR reactor, can be measured.

In spite of advances, there continues to be a long felt need for anaffordable and reliable diesel exhaust aftertreatment system that isdurable, has a manageable operating cost (including fuel penalty), andreduces NO_(x) emissions to a satisfactory extent in the sense ofmeeting U.S. Environmental Protection Agency (EPA) regulations effectivein 2010 and other such regulations.

SUMMARY

A difficulty of scheduling LNT regenerations based on NO_(x)concentration measurements is that this data can be chaotic. Events suchas gear changes can cause brief transient increases in exhaust NO_(x)concentrations downstream from an LNT. The variability of the data iseven greater if differential between the NO_(x) concentration enteringthe LNT and the NO_(x) leaving the LNT is calculated in order toestimate the NO_(x) removal efficiency.

The inventors have observed that NO_(x) concentration sensor data isparticularly chaotic and unreliable in periods immediately following LNTregeneration events, but subsequently becomes more stable. Accordingly,one of the inventors' concepts is a method of operating a diesel powergeneration system that comprises treating the diesel exhaust with anexhaust aftertreatment system comprising an LNT and denitrating the LNTselectively based on data from a NO_(x) sensor wherein the decision todenitrate ignores data from the NO_(x) sensor or places decreased weighton data from that sensor obtained in a period immediately following adenitration. The period immediately follows the preceding denitration.The weight is decreased as compared to the weight placed on dataobtained from that sensor after the period. This method reducespremature denitrations and associated fuel expenditures resulting frommisleading NO_(x) concentration data.

Another of the inventors concepts' relates to selectively denitratingthe LNT based on both a first and a second criteria being met. The firstcriteria relates to the amount of NO_(x) stored in the LNT or remainingNO_(x) storage capacity of the LNT. The second criteria relates to thecurrent performance of the exhaust treatment system, or a portionthereof, as determined from one or more measurements of NO_(x)concentration in the exhaust. This method reduces premature denitrationsand associated unnecessary fuel expenditures resulting from inaccurateNO_(x) concentration data and transient events.

The inventors have found that over a typical course of operating adiesel engine opportunities for regenerating at relatively low fuelpenalty routinely occur. Advantage of these opportunities can be takenby weighing in the decision to regenerate both the state and orperformance of the aftertreatment system comprising the LNT and theconduciveness of current conditions toward regenerating the LNT with alow fuel penalty. According, the inventors conceived a method ofoperating a power generation system comprising operating a diesel engineto produce a lean exhaust comprising NO_(x), passing the lean exhaustthrough an exhaust aftertreatment system comprising an LNT that adsorbsa portion of the NO_(x) from the exhaust, and selectively denitratingthe LNT based on a balance between the state and or performance of theaftertreatment system and the conduciveness of current conditions toregenerating the LNT with a low fuel penalty. The method results inregeneration being advanced when conditions are favorable and postponedwhen conditions are not favorable, with the ultimate result of greaterfuel economy for a given level of emission control. The balance varies athreshold for regeneration.

A threshold for regeneration is a point at which a measure of the stateand or performance of the exhaust aftertreatment system comprising theLNT reaches a critical value. In one embodiment, the balancing involvessetting the critical value based on conduciveness. In anotherembodiment, a first factor relating to conduciveness is determined, asecond factor relating to urgency of the need to regenerate (dependingon state or performance) is determined, and the two factors weighed in aformula that determines whether the time to regenerate has arrived. Inthis later embodiment, a critical value exists but may never beexplicitly calculated.

The inventors also recognize that the urgency of the need to regeneratedoes not have a static relationship to the state or performance of theexhaust aftertreatment system. The performance requirements for theexhaust aftertreatment system in general and the LNT in particular arenot static, but a variable function of the vehicle operating state. Forsome vehicle operating states the LNT must be maintained at a relativelylow level of saturation in order to adequately control NOx emissions. Inother vehicle operating states, the LNT continues to meet emissioncontrol objectives even at relatively high levels of saturation. Thus,if LNT denitration criteria remains a fixed function of either NOxloading, remaining NOx storage capacity, or NOx mitigation efficiency,regeneration will either be overly frequent and wasteful of fuel orinsufficiently frequent to achieve adequate NOx mitigation. Accordingly,the inventors conceive a method of operating a power generation systemcomprising operating a diesel engine to produce a lean exhaustcomprising NOx, passing the lean exhaust through an exhaustaftertreatment system comprising a lean NOx trap that adsorbs a portionof the NOx from the exhaust, and selectively denitrating the lean NOxtrap based on a measure relating to NO_(x) loading, remaining NO_(x)storage capacity, or performance of the exhaust aftertreatment system ora portion thereof comprising the lean NO_(x) trap. A critical value forthe measure is determined based in part on the vehicle operating state,whereby regeneration is advanced when demands on the aftertreatmentsystem are high and postponed when demands on the aftertreatment systemare comparatively low.

Another of the inventors' concepts is to select the critical value basedon engine speed-load information. Engine speed-load information cancomprise one or more of the engine's current speed, the engine's currentload, a gradient in the engine speed, a gradient in the engine load, andother historical information regarding the engines speed and/or load. Ingeneral, regeneration is triggered when a measure reaches a criticalvalue. The measure relates to the state or performance of an exhaustaftertreatment system comprising an LNT, or a portion thereof. Asdescribed above, at least two of the inventors' concepts involve varyingthe critical value. In one concept the critical value is varied based onthe conduciveness of current conditions to regeneration. In anotherconcept, the critical value is varied based on the demands being placedon the exhaust aftertreatment system. Both conduciveness of currentconditions to regeneration and demands placed on the aftertreatmentsystem depend primarily on the engine current speed-load point and theengine's speed-load history, although other factors may be relevant.Nevertheless, because of the important role and ready accessibility ofthe engine speed-load information, another of the inventors' concepts isto select the critical value based on that information.

The various concepts and methods described in this summary can be usedseparately or two or more together to improve denitration schedulingover the prior art, thus meeting emission control targets whilelessening the associated fuel penalty.

The primary purpose of this summary has been to present certain of theinventors' concepts in a simplified form to facilitate understanding ofthe more detailed description that follows. This summary is not acomprehensive description of every one of the inventors' concepts orevery combination of the inventors' concepts that can be considered“invention”. Other concepts of the inventors will be conveyed to one ofordinary skill in the art by the following detailed description togetherwith the drawings. The specifics disclosed herein may be generalized,narrowed, and combined in various ways with the ultimate statement ofwhat the inventors claim as their invention being reserved for theclaims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exhaust aftertreatment systemthat can embody various control concepts described herein.

FIG. 2 is a flow chart of a control process that ignores sensor data ina period following denitration.

FIG. 3 is a flow chart of a control process in which regeneration doesnot proceed unless two different criteria are satisfied.

FIG. 4 is a flow chart of a control process in which the urgency of theneed to regenerate is weighed against the conduciveness of conditions todenitration.

FIG. 5 is a flow chart of a control process in which a threshold forregeneration is set based on the demands being placed on the exhaustaftertreatment system.

FIG. 6 is a flow chart of an exemplary control process implementingseveral of the inventors' concepts.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a power generation system 100 towhich various of the inventors' concepts are applicable. The powergeneration system 100 is not the only power generation system to whichthe inventors' concepts are applicable, but the various conceptsdescribed herein were originally developed for systems like the system100 and the individual components of the system 100 pertain to preferredembodiments. The power generation system 100 comprises a diesel engine101 and an exhaust line 102 in which are configured components of anexhaust aftertreatment system 103. The exhaust aftertreatment system 103comprises a fuel reformer 104, a lean NO_(x) trap 105, and anammonia-SCR catalyst 106. A fuel injector 107 is configured to injectfuel into the exhaust line 102 upstream from the fuel reformer 104. Acontroller 108 controls the fuel injection based on information aboutthe engine 101, a temperature of the fuel reformer 104 measured by atemperature sensor 109, and a NO_(x) concentration measurement obtainedby the NO_(x) sensor 110 at a point in the exhaust line 102 downstreamfrom the lean NO_(x) trap 105. A temperature sensor 111 is configured tomeasure the temperature of the lean NO_(x) trap 105, which isparticularly important during desulfation.

The diesel engine 101 is a compression ignition engine. A compressionignition diesel engine normally produces exhaust having from about 4 toabout 21% O₂. An overall rich exhaust-reductant mixture can be formed byinjecting diesel fuel into the exhaust during cylinder exhaust strokes,although it is preferred that any reductant be injected into the exhaustdownstream from the engine 101. The engine 101 is commonly provided withan exhaust gas recirculation (EGR) system and may also be configuredwith an intake air throttle, either of which can be used to reduce theexhaust oxygen concentration and lessen the amount of reductant requiredto produce an overall rich exhaust-reductant mixture. A lean burngasoline engine or a homogeneous charge compression ignition engine canbe used in place of the engine 101. The engine 101 is operative toproduce an exhaust that comprises NO_(x), which is considered to consistof NO and NO₂.

The engine 101 is generally a medium or heavy duty diesel engine. Theinventors' concepts are applicable to power generation systemscomprising light duty diesel and lean burn gasoline engines, but theperformance demands of exhaust aftertreatment systems are generallygreater when the engine is a medium or heavy duty diesel engine. Minimumexhaust temperatures from lean burn gasoline engines are generallyhigher than minimum exhaust temperatures from light duty diesel engines,which are generally higher than minimum exhaust temperatures from mediumduty diesel engines, which are generally higher than minimum exhausttemperatures from heavy duty diesel engines. Lower exhaust temperaturesmake NO_(x) mitigation more difficult and fuel reformers harder to lightoff. A medium duty diesel engine is one with a displacement of at leastabout 4 liters, typically about 7 liters. A heavy duty diesel engine isone with a displacement of at least about 10 liters, typically fromabout 12 to about 15 liters.

The exhaust from the engine 101 is channeled by a manifold to theexhaust line 102. The exhaust line 102 generally comprises a singlechannel, but can be configured as several parallel channels. The exhaustline 102 is preferably configured without exhaust valves or dampers. Inparticular, the exhaust line 102 is preferably configured without valvesor dampers that could be used to vary the distribution of exhaust amonga plurality of LNTs 105 in parallel exhaust channels. The inventors'concepts are applicable to aftertreatment systems with exhaust valves ordampers, but the presence of exhaust valves or dampers can considerablyalter the consideration that go into scheduling LNT regeneration. Valvesor dampers can be used to reduce the exhaust flow to a fuel processor orLNT, allowing regeneration to be carried out efficiently even whenexhaust conditions are unfavorable. Nevertheless, it is preferred thatthe exhaust line 102 be configured without valves or dampers becausethese moving parts are subject to failure and can significantly decreasethe durability and reliability of an exhaust aftertreatment system.

Even when the exhaust line 102 is free from exhaust valves or dampers,an exhaust line upstream from the exhaust line 102 may still contain anexhaust valve, such as an exhaust gas recirculation (EGR) valve in anEGR line. Exhaust valves are particularly problematic when they areconfigured within a main exhaust line to divert a majority of theexhaust flow as compared to exhaust valves configured to control theflow through a side branch off a main exhaust line. Exhaust valves forlarger conduits are more subject to failure than exhaust valves forsmaller conduits.

The exhaust line 102 is provided with an exhaust line fuel injector 107to create rich conditions for regenerating the LNT 105. The inventors'concepts are applicable to other method's of creating a reducingenvironment for regenerating the LNT 105, including engine fuelinjection of reductant and injection of reductants other than dieselfuel. Nevertheless, it is preferred that the reductant is the samediesel fuel used to power the engine 101. It is also preferred that thereductant be injected into the exhaust line 102, rather than into thecylinders of engine 101, in order to avoid oil dilution caused by fuelpassing around piston rings and entering the oil gallery. Additionaldisadvantages of cylinder reductant injection include having to alterthe operation of the engine 101 to support LNT regeneration, excessivedispersion of pulses of reductant, forming deposits on any turbochargerconfigured between the engine 101 and the exhaust line 102, and formingdeposits on any EGR valves.

The diesel fuel is injected into the exhaust line 102 upstream from afuel reformer 104. The fuel reformer 104 comprises an effective amountof precious metal catalyst, including rhodium, to catalyze oxidation andsteam reforming reactions at 600° C. The fuel reformer 104 is designedwith low thermal mass, whereby it can be easily heated to steamreforming temperatures for each LNT regeneration. Low thermal mass istypically achieved by constructing the fuel reformer 104 around a thinmetal substrate. A thin metal substrate has a thickness that is about100 μm or less, preferably about 50 μm or less, and still morepreferably about 30 μm or less.

Steam reforming temperatures are at least about 500° C., which isgenerally above diesel exhaust temperatures. Diesel exhaust temperaturesdownstream from a turbocharger vary from about 110 to about 550° C.Preferably, the fuel reformer 104 can be warmed up and operated usingdiesel fuel from the injector 107 stating from an initial temperature of275° C. while the exhaust from the engine 101 remains at 275° C. Morepreferably, the fuel reformer 104 can be warmed up and operated frominitial exhaust and reformer temperatures of 225° C., and still morepreferably from exhaust and reformer temperatures of 195° C. Theseproperties are achieved by providing the fuel reformer 104 witheffective amounts of precious metals, such as Pt and/or Pd, forcatalyzing oxidation of diesel fuel at the starting temperatures. Lowtemperature start-up can also be improved by configuring a low thermalmass precious metal oxidation catalyst upstream from the fuel reformer104. Preferably, the upstream catalyst combusts a portion of the fuelwhile vaporizing the rest. A mixing zone between the upstream catalystand the fuel reformer 104 is also helpful.

Having the fuel reformer 104 operate at steam reforming temperaturesreduces the total amount of precious metal catalyst required. Lessprecious metal catalyst is required when reforming at steam reformingtemperatures as compared to reforming diesel fuel at exhausttemperatures regardless of whether reforming is through partialoxidation and stream reforming or exclusively though partial oxidationreactions.

Having the fuel reformer 104 operate at least partially through steamreforming reactions significantly increases the reformate yield andreduces the amount of heat generation. In principal, if reformateproduction proceeds through partial oxidation reforming as in thereaction:CH_(1.85)+0.5O₂→CO+0.925H₂  (1)1.925 moles of reformate (moles CO plus moles H₂) could be obtained fromeach mole of carbon atoms in the fuel. CH_(1.85) is used to representdiesel fuel having a typical carbon to hydrogen ratio. If reformateproduction proceeds through steam reforming as in the reaction:CH_(1.85)+H₂O→CO+1.925H₂  (2)2.925 moles of reformate (moles CO plus moles H₂) could in principle beobtained from each mole of carbon atoms in the fuel. In practice, yieldsare lower than theoretical amounts due to the limited efficiency ofconversion of fuel, the limited selectivity for reforming reactions overcomplete combustion reactions, the necessity of producing heat to drivesteam reforming, and the loss of energy required to heat the exhaust.

Preferably, the fuel reformer 104 comprises enough steam reformingcatalyst that at 600° C., with an 8 mol % exhaust oxygen concentrationfrom the engine 101 and with sufficient diesel fuel to provide theexhaust with an overall fuel to air ratio of 1.2:1, at least about 2 mol% reformate is generated by steam reforming, more preferably at leastabout 4 mol %, and still more preferably at least about 6 mol %. Forpurposes of this disclosure, functional descriptions involving dieselfuel are tested on the basis of the No. 2 diesel fuel oil sold in theUnited States, which is a typical diesel fuel.

The inventors' concepts are applicable to power generation systems thatdo not process injected diesel fuel through fuel reformers comprisingsteam reforming catalysts. For example, the injected diesel fuel can becombusted to consume excess oxygen in the LNT 106, or in an upstreamoxidation catalyst. The injected diesel fuel can also be processed toform reformate by partial oxidation reactions below steam reformingtemperatures.

An LNT is a device that adsorbs NO_(x) under lean conditions and reducesand releases NO_(x) under rich conditions. An LNT generally comprises aNO_(x) adsorbent and a precious metal catalyst in intimate contact on aninert support. Examples of NO_(x) adsorbent materials include certainoxides, carbonates, and hydroxides of alkaline earth metals such as Mg,Ca, Sr, and Ba or alkali metals such as K or Cs. The precious metaltypically consists of one or more of Pt, Pd, and Rh. The support istypically a monolith, although other support structures can be used. Themonolith support is typically ceramic, although other materials such asmetal and SiC are also suitable for LNT supports. The LNT 105 may beprovided as two or more separate bricks.

The ammonia-SCR catalyst 106 is functional to catalyze reactions betweenNO_(x) and NH₃ to reduce NO_(x) to N₂ in lean exhaust. The ammonia-SCRcatalyst 106 adsorbs NH₃ released from the LNT 105 during denitrationand subsequently uses that NH₃ to reduce NO_(x) slipping from the LNT105 under lean conditions. Examples of ammonia-SCR catalysts includecertain oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni,Mo, W, and Ce and zeolites, such as ZSM-5 or ZSM-11, substituted withmetal ions such as cations of Cu, Co, Ag, or Zn. Ammonia-SCR can beaccomplished using precious metals, but preferably the SCR catalyst 106is substantially free of precious metals. Preferably, the ammonia-SCRcatalyst 106 is designed to tolerate temperatures required to desulfatethe LNT 105.

The exhaust aftertreatment system 100 can comprise other components,such a diesel particulate filter and a clean-up oxidation catalyst. Athermal mass can be placed between the fuel reformer 104 and the LNT 105to protect the LNT 105 from frequent exposure to high fuel reformertemperatures. A diesel particulate filter can be used as the thermalmass.

During normal operation (a lean phase), the engine 101 is operated toproduce an exhaust comprising NO_(x), particulate matter, and excessoxygen. A portion of the NO_(x) is adsorbed by the LNT 105. Theammonia-SCR catalyst 106 may have ammonia stored from a previousdenitration of the LNT 105. If the ammonia-SCR catalyst 106 containsstored ammonia, an additional portion of the NO_(x) is reduced over theammonia-SCR catalyst 106 by reaction with stored ammonia. The fuelinjector 107 is generally inactive over this period, although small fuelinjections might be used to maintain the fuel reformer 104 at atemperature from which it can be easily heated or to maintain the leanNO_(x) trap 105 at a temperature at which it effectively absorbs NO_(x).

From time-to-time, the LNT 105 must be regenerated to remove accumulatedNO_(x) (denitrated) in a rich phase. Denitration generally involvesheating the reformer 104 to an operational temperature and then usingthe reformer 104 to produce reformate. The reformer 104 is generallyheated by injecting fuel into the exhaust upstream from the fuelreformer 104 at a sub-stoichiometric rate, whereby the exhaust-reductantmixture remains overall lean and most of the injected fuel completelycombusts in the reformer 104. This may be referred to as a lean warm-upphase. Once combustion has heated the reformer 104, the fuel injectionrate can be increased and/or the exhaust oxygen flow rate reduced tomake the exhaust-reductant mixture overall rich, whereupon the reformer104 consumes most of the oxygen from the exhaust and produces reformateby partial oxidation and/or steam reforming reactions. The reformatethus produced reduces NO_(x) absorbed in the LNT 105. Some of the NO_(x)may be reduced to NH₃, which is absorbed and stored by the ammonia-SCRcatalyst 106.

Most of the inventors' concepts described herein relate to schedulingthe start of denitration. The scheduling is carried out by thecontroller 108, which provides a control signal once the criteria forinitiating a denitration have been met. The controller 108 may alsoprovide a control signal once criteria marking the end of denitrationhave been met. The controller 108 may be an engine control unit or maybe a separate device.

From time to time, the LNT 105 must also be regenerated to removeaccumulated sulfur compounds (desulfated). Desulfation involves heatingthe fuel reformer 104 to an operational temperature, heating the LNT 105to a desulfating temperature, and providing the heated LNT 105 with arich atmosphere. Desulfating temperatures vary, but are typically in therange from about 500 to about 800° C., with optimal temperaturestypically in the range of about 650 to about 750° C. Below a minimumtemperature, desulfation is very slow. Above a maximum temperature, theLNT 105 may be damaged.

The LNT 105 is heated in part by heat convection from the reformer 104.To generate this heat, fuel can be supplied to the reformer 104 underlean conditions, whereby the fuel combusts in the reformer 104. Once thereformer 104 is heated, the fuel injection rate can be controlled tomaintain the temperature of the reformer 104 while the LNT 105 areheating.

Treatment of NO_(x) Sensor Data

The controller 108 uses data from the NO_(x) sensor 110 to scheduledenitrations for the LNT 105. It is possible to schedule denitrationswithout using a NO_(x) sensor positioned downstream from the LNT 105,but the use of such a sensor is widely considered desirable. The sensorcan be used to track whether emission control objectives are being met,to measure the current efficiency of the LNT 105, and/or to determinehow much NO_(x) is currently being removed from the exhaust by the LNT105 as part of a calculation of how much NO_(x) the LNT 105 has adsorbedsince its last denitration.

It is known that sensors do not provide strictly accurate informationand it is usual to process sensor data in various ways prior to using itin control algorithms. The data from the NO_(x) sensor 110 can beinaccurate due to random perturbations and noise that affects themeasurements. Data is also inaccurate in that it is not current due tolag in obtaining measurements. In general, the data is processed througha state estimator, which can be any algorithm used to determine thestate of a system from data. A state estimator can be a simple filter orit can use a model of the system, in which case it may be referred to asan observer. State estimators can operate in numerous ways, e.g.,providing moving time averages, discarding outliers in a sequence ofmeasurements, and/or estimating the state in a manner that balances whatcan be expected based on a model with what the sensor is asserting. Thestate in this case is a NO_(x) concentration downstream from the LNT105, although the state can be more complex than a single NO_(x)concentration. The NO_(x) concentration can be either upstream ordownstream of the ammonia-SCR catalyst 106.

The inventors have found that a typical NO_(x) sensor downstream from anLNT, is particularly unreliable in periods immediately following therich phases used to regenerate the LNT. After some amount of leanoperation, the sensor data again become more reliability. The reason forthe periods of unreliability is not known. The inventors have determinedthat control using a NO_(x) sensor downstream from an LNT is improved ifthe sensor data is ignored or at least given reduced weight in a periodimmediately following a rich regeneration.

In one embodiment, the data from the NO_(x) sensor from a periodimmediately following a denitration is ignored or discarded. The periodmay be fixed or variable, predetermined or event-based. A variableperiod could be based, for example, on the time taken for the engine 101to use a certain amount of fuel or generate a certain volume of exhaust.An event-based period could be a period that ends based on anobservation of the NO_(x) sensor data indicating the NO_(x) sensor datais again reliable. If the decision to begin denitration is based on aprogressive calculation made using the NO_(x) sensor data, such as acalculation of the amount of NO_(x) taken up by the LNT 105, estimatescan be used in place of the data over the period. If the decision tobegin denitration can be triggered by a single NO_(x) sensor reading,the trigger is prevented over the period.

FIG. 2 provides a flow chart of a control process 200 providing anexemplary implementation of this method. Block 201 makes the decisionwhether to initiate denitration based on sensor data. Once the decisionis made, the process proceeds to block 202 where denitration is carriedout. Following denitration, rather than immediately returning to block201, the process dwells in block 203, allowing time to pass until thesensor data can again be considered sufficiently reliable. After waitingfor a period in block 203, the process returns to the decision block201.

In another embodiment, data from the NO_(x) sensor 110 is given reducedweight over the period. Reduced weight can be implemented through anobserver, for example. An observer can include a parameter related to anestimate of the contribution of noise to the sensor measurement. Such aparameter can be altered to reflect the greater contribution of noiseover the period, whereby some useful information can be salvaged.Subsequent to the period, the weight given to the NO_(x) sensor data asit relates to a decision whether to begin denitrating is againincreased.

Regenerating Only when Two Criteria are Met

Another concept that addresses the general issue of NO_(x) sensor datareliability is to require that two separate criteria be met beforeinitiating a denitration. A first criteria relates to the amount ofNO_(x) stored in the lean NO_(x) trap 105 or remaining NO_(x) storagecapacity of lean NO_(x) trap 105. The second criteria relates to thecurrent performance of the exhaust aftertreatment system 103, or aportion thereof, as determined from one or more measurements of NO_(x)concentration in the exhaust. The first criteria ensures thatdenitration will not begin prematurely due to noise or brief eventsaffecting the NO_(x) sensor data. Requiring that the second criteriaalso be satisfied before beginning denitration allows the lean phases tobe extended beyond the conservative limits that must be used if thefirst criteria is the only criteria.

Extending the lean phases when permissible based on the second criteriareduces the frequency of regeneration, which allows emission controltargets to be reached at a reduced fuel penalty. Fuel penalty is reducedin that reductants are used more efficiently when NO_(x) loading ishigh. Fuel penalty is also reduced in that there is generally a start-upfuel penalty for beginning denitrations; reducing the frequency ofdenitration reduces the number of start-up penalties that are incurred.Finally, with the SCR catalyst 106 configured downstream from the LNT105, fuel penalty is reduced in that ammonia production is greater whendenitration is postponed to higher NO_(x) loading. Ammonia removesNO_(x) from the exhaust by reaction over the SCR catalyst 106 withoutfuel penalty.

The first criteria relates to the accumulation of NO_(x) in the LNT 105.The accumulation of NO_(x) in the LNT 105 can be characterized either interms of the amount of NO_(x) stored in the LNT 105 or the amount ofNO_(x) storage capacity remaining in the LNT 105. Either case generallyinvolves estimating and integrating the rate at which NO_(x) is beingstored in the LNT 105 and estimating the storage capacity of the LNT105. Amounts can be expressed as percentages of capacity. Saturation isNO_(x) loading as a fraction of a theoretical loading limit.

Any suitable method can be used to estimate the rate at which NO_(x) isbeing stored in the LNT 105. In one embodiment, the rate of accumulationis calculated by multiplying the volumetric flow rate of the exhaust bythe difference between the NO_(x) concentration upstream from the LNT105 and the NO_(x) concentration downstream from the LNT 105. This isthe preferred method if the NO_(x) sensor 110 is located upstream fromthe SCR catalyst 106. Preferably, the volumetric flow rate of theexhaust and the NO_(x) concentration upstream from the LNT 105 areestimated from an engine map, whereby sensors are not required todetermine these quantities.

In another embodiment, the rate of accumulation is calculated bymultiplying the flow rate of NO_(x) into the LNT 105 by an estimate ofthe LNT 105's NO_(x) removal efficiency. This is the preferred method ifthe NO_(x) sensor 110 is downstream from the SCR catalyst 106. Theefficiency of the LNT 105 can be estimated from a model. A model cantake into account one or more variables affecting LNT efficiency such asthe LNT temperature, LNT NO_(x) loading, LNT SO_(x) loading, and exhaustflow rate.

The second criteria relates to current performance of the exhaustaftertreatment system 103 as determined using data from the NO_(x)sensor 110. The performance can be of the LNT 105 alone or of the LNT105 in combination with the SCR catalyst 106. Any suitable performancemeasure can be used. Examples of potentially suitable performancemeasures include the concentration to which NO_(x) is reduced, thefraction of NO_(x) removed from the exhaust by one or moreaftertreatment devices including the LNT 105, and the NO_(x) emission orslip rate. The NO_(x) emission rate can be normalized. Normalizing withrespect to the engine bhp-hr makes the measure relate directly tocertain emission control standards.

NO_(x) removal efficiency measurements can also be normalized. In oneembodiment, the efficiency is normalized for exhaust flow rate. Exhaustflow rate has a strong effect on observed NO_(x) removal efficiency. Anefficiency measurement normalized for exhaust flow rate gives a moreaccurate reading of the condition of the exhaust aftertreatment system103, or a portion thereof, than a measure that is not so normalized.Likewise, an efficiency measure can be normalized for temperature.Normalizing for temperature is done based on an expression that givesthe temperature dependence of the NO_(x) removal efficiency. By removingthe effect of temperature dependence, the efficiency measure relatesmore directly to the effect of the current NO_(x) loading level on theefficiency of the LNT 105.

Denitration, once begun, can proceed to any suitable endpoint.Denitration can extend for a fixed period or until a fixed amount ofreductant has been supplied. Optionally, the length of denitration isbased at least in part on the estimated NO_(x) loading of the LNT 105.In one embodiment, denitration proceeds to reduce the NO_(x) loading toa target level. The target level can be fixed or variable. An example ofa variable target would be one that depends on the NO_(x) loading at thebeginning of the denitration. If denitration was begun at acomparatively low NO_(x) loading due to the criteria relating to currentperformance of the exhaust aftertreatment system 103 being metrelatively early, it may be desirable to denitrate to a greater degreein order to keep the regeneration frequency relatively low.

FIG. 3 provides a flow chart of an exemplary control process 300implementing this concept. The process begins in a lean phase whereNO_(x) is accumulating in the LNT 105. Block 301 checks whether the LNT105 has become saturated with NO_(x) to a critical level. If yes, theprocess proceeds to block 302, which checks whether the LNT efficiencyhas fallen below a limit. With the second yes, the process proceeds toblock 303 where denitration is carried out. The estimate of LNTsaturation used in block 301 is reduced to a degree commensurate withthe extent of denitration. Denitration also restores the LNT efficiency.When denitration is complete, the process returns to a lean phase andblock 301.

The concept of beginning a regeneration only when two criteria are met,one relating to current performance of the LNT 105 and the otherrelating to loading or remaining capacity, is also applicable todesulfation. SO_(x) loading can be estimated based on fuel usage sincethe last desulfation, optionally modified by an SO_(x) uptake efficiencyfor the LNT 105. The current performance as it relates to sulfur loadingcan be determined in any suitable manner. One measure of performanceaffected by sulfur loading is the frequency with which regeneration isrequired. Another measure of current performance as it relates to sulfurloading is NO_(x) removal efficiency of the LNT 105 as measured shortlyafter a thorough denitration. Another measure of current performance asit relates to sulfur loading is based on the relationship between NO_(x)removal efficiency and NO_(x) loading; if after taking into account theeffect of current conditions such as LNT temperature and exhaust flowrate it is apparent the NO_(x) removal efficiency is low in comparisonto what would be expected from the LNT 105 in a desulfated condition atthe current level of NO_(x) loading, the need for desulfation isindicated.

Balancing Urgency of the Need to Regenerate Against Conduciveness

Another of the inventors' concepts is to balance the need forregeneration against the conduciveness of current conditions toregeneration, whereby early regeneration is favored when conditions areconducive. If conduciveness to regeneration were always the same, itwould be desirable to postpone denitration until denitration becameimperative. The inventors have found, however, that for typical vehicleoperation exhaust conditions change frequently and dramatically, wherebya regeneration strategy that ignores changing conditions is far fromoptimal. From time-to-time, conditions that are highly favorable toregeneration occur making early regeneration preferable to postponingregeneration until NO_(x) loading of the LNT 105 is higher. Likewise,particularly unfavorable conditions may occur from time-to-time. At somepoint, it becomes preferable to regenerate earlier than necessary toreduce the risk that regeneration will be required under particularlyunfavorable conditions that might subsequently occur.

The balancing determines a threshold for regeneration. A threshold is apoint where some measure of the state and or performance of an exhaustaftertreatment system, or some portion thereof comprising an LNT,reaches a critical value at which regeneration is triggered. Thethreshold can be assigned one of several possible points based theconditions that determine favorability. Preferably, however, thethreshold is varied according to a continuous function of some measureof the favorability of conditions.

The threshold defines a criteria that is met if some measure of theexhaust aftertreatment system 103's, or a portion thereof's, state orperformance has reached a critical value. The measure relates to theurgency of the need to regenerate. Any suitable measure can be used inthe criteria. The measure can relate to current performance of theexhaust aftertreatment system 103 as determined using data from theNO_(x) sensor 110. Alternatively, without limitation, the measure canrelate to the accumulation of NO_(x) in the LNT 105.

Determining whether the criteria has been met and the threshold crossedcan be done in any suitable fashion. In one example, the determinationis made by comparing the measure of the exhaust aftertreatment system103's state or performance to the threshold value. In another example,one value is generated based on the conduciveness of conditions toregeneration, another value is generated based on the urgency of theneed to regenerate, and the determination made by weighing the twovalues against one another in a formula (balancing the factors). In thelater example, a threshold value the exhaust aftertreatment system 103'sstate or performance may not be explicitly determined, although such avalue exist in a mathematical sense.

The urgency of the need to regenerate can be related to a comparisonbetween a measure of the exhaust aftertreatment system 103's state orperformance and a critical value at which regeneration is consideredimperative. The urgency can be placed on a scale beginning from wherethe urgency is at its lowest and ending where regeneration is imperativeto avoid violating an emission limit. For example, if the measure ofurgency is a NO_(x) emission rate in terms of g/bhp-hr, the urgency canbe assigned a value of zero when the emission rate is zero and one whenthe emission rate is at a mandated not-to-exceed limit, the urgencyvarying linearly for values between.

As used herein, conduciveness of conditions to regeneration assumes thatregeneration is possible and focuses on the fuel penalty forregeneration. Accordingly, conduciveness of conditions to regenerationis determined by considering one or more conditions that affect the fuelpenalty for regeneration. Some of the most important conditions thataffect the fuel penalty include the exhaust oxygen flow rate, theexhaust oxygen concentration, and the exhaust flow rate. Low oxygen flowrate means less reductant is consumed removing oxygen from the exhaustover the course of regeneration. Low oxygen concentration relates to lowoxygen flow rate, but also relates to the ability to operate the fuelreformer 104 with little or no fuel pulsing. When oxygen concentrationsare high, pulsed operation may be necessary to prevent overheating. Lowexhaust flow rate also relates to low oxygen flow rate, but also relatesto efficient reductant usage due to long residence times. Potentialexhaust conditions achievable by throttling the engine air intake orchanging the transmission gear ratios while maintaining the engine'spower output can be determined from current conditions and canthemselves be considered current conditions.

The conduciveness of conditions to regeneration can be related to theengine speed-load state information. At steady state, exhaust conditionssuch as flow rate and oxygen concentration are related to the engine'sspeed-load point. Accordingly, the engine speed-load point can be usedto estimate conduciveness of conditions to regeneration. Duringtransient operation, the exhaust conditions depend on the speed-loadhistory as well as the current speed-load. In particular, it has beenobserved that a favorable combination of low exhaust flow rate and lowexhaust oxygen concentrations often occur during positive speedgradients and positive load gradients. Thus, if conduciveness ofconditions to regeneration is determined from speed-load stateinformation, it is preferably that speed and load gradient be consideredin addition to the engine 101's current speed-load point.

Conduciveness can be placed on a scale beginning where conditions areleast conducive and ending where conditions are most conducive. Forexample, a value of zero can be assigned to the condition where theexhaust oxygen flow rate is at its highest and a value of one can beassigned to the condition where the exhaust oxygen flow rate is at itslowest (for a running engine), with the numerical value measuringconduciveness varying linearly in between.

Balancing or the urgency and conduciveness factors can take place in anysuitable manner. For example, the numerical factors can be added ormultiplied together, and the result checked to determined whether athreshold to begin regeneration has been reached. Equivalently, theconduciveness factor can be used to set a critical value against whichthe urgency measure is checked. For example, where the urgency measureis based on NO_(x) emission rate in terms of g/bhp-hr, the criticalemission rate after which regeneration is initiated can be reduced inproportion to the magnitude of the conduciveness factor. As anotherexample, where the urgency measure is based on the NO_(x) storagecapacity remaining in the LNT 105, the capacity threshold at whichdenitration begins can be increased in proportion to the magnitude ofthe conduciveness factor, whereby the threshold is reached more quicklyif the conduciveness is high.

Denitration, once begun, can proceed to any suitable endpoint.Denitration can proceed to a fixed endpoint or to an extent that dependsonly the factors affecting urgency of the need to regenerate.Optionally, however, the length of denitration is based in part on theon the conduciveness of conditions to regeneration. Preferably, thedenitration proceeds to a greater extent if conditions are conducive todenitration and to a lesser extent if conditions are not conducive. Theendpoint target can be revised as conditions change over the course of adenitration. For example, if conditions worsen, denitration can be endedearlier. An endpoint for regeneration can be defined in any suitablefashion, but typically the endpoint is determined from an estimate ofhow much NO_(x) has been removed from the LNT 105 or how much NO_(x)remains in the LNT 105.

FIG. 4 provides a flow chart of an exemplary control method 400 in whichconduciveness of conditions to regeneration is balanced against theurgency of the need to regenerate. The process begins in a lean phase inblock 401. In block 401, the urgency of the need to regenerate isassigned a numerical value. For example, the urgency can be based on aNO_(x) removal efficiency of the aftertreatment system 103 and assigneda value based on a current estimate of the NO_(x) removal efficiency insuch a way that the value increases and approaches 1.0 as the minimumacceptable efficiency is approached.

In block 402, a numerical value is assigned to the conduciveness ofcurrent conditions to regeneration. For example, the conduciveness canbe based on the exhaust oxygen flow rate and can be assigned in such away that the value is zero when the exhaust oxygen flow rate is at itshighest and the value is 0.7 when the exhaust oxygen flow rate is at itslowest.

In block 403 the numerical values assigned to conduciveness and urgencyare combined into a single factor that can be tested to determinewhether to regenerate. In this example, the numerical values are simplyadded together to generate the factor.

The factor is tested in block 404 to determined whether to initiateregeneration. For example, the test can be whether the factor exceeds1.0. If yes, denitration is begun in block 405. If no, the methodreturns to repeat blocks 401-404.

Regenerating Earlier when Demands Placed on the Emission Control Systemare Greater

The inventors have also recognized that for measures of the urgency ofthe need to regenerate that relate only to the state of the exhaustaftertreatment system 103 and for many measures of the performance ofthe exhaust aftertreatment system 103 the urgency is less than indicatedby the measure for some operating conditions than for others.Accordingly, another of the inventors' concepts is to balance a measureor the state or performance of the exhaust aftertreatment system 103, ora portion thereof, with the demands currently being placed on theexhaust aftertreatment system 103 by a vehicle operating state in makingthe decision whether to regeneration. The balancing affects a criticalvalue for the measure of state or performance at which regeneration isinitiated.

The critical value that is changed based on conditions can be associatedwith any of the usual criteria used to determine when to initiate an LNTregeneration. For example, the criteria can relate to currentperformance of the exhaust aftertreatment system 103 as determined usingdata from the NO_(x) sensor 110, to the accumulation of NO_(x) in theLNT 105, or to remaining NO_(x) storage capacity in the LNT 105.

The demands being placed on the aftertreatment system 103 are primarilya function of the operating state of the engine 101. The demands aregenerally greater when the exhaust flow rate is greater, when the engine101 is producing more NO_(x), or when the engine 101 is producing anexhaust outside the optimal temperature range for the treatment byexhaust aftertreatment system 103. At higher exhaust flow rates,residence times are shorter and a greater activity of the devices in theexhaust treatment system 103 is required to achieve a target degree ofemission control. When the engine 101 is producing more NO_(x),particularly when the engine 101 is producing more NO_(x) on a g/bhp-hrbasis, a greater fraction of the NO_(x) must be removed from the exhaustby the exhaust aftertreatment system 103. When the exhaust cools orheats the LNT 105 to outside its optimal operating temperature range,the LNT 105's effectiveness becomes unacceptably low at a lower NO_(x)loading than when the LNT 105 is within an optimal temperature range.

Optionally, the demands on the aftertreatment system can be related toan engine map. In general, demands are greatest when the engine 101 isin a high-speed, high-load condition. It is desirable to triggerregeneration sooner when the engine 101 is in a high-speed high-loadcondition, whereas regeneration can be postponed until NO_(x) loading isgreater and the performance of the LNT 105 has declined to a greaterdegree if less demanding conditions prevail. The demands also relate tothe recent speed-load history, as may be characterized by the speed-loadgradient, however, it is less useful to consider such transient effectswhen evaluating current demands. Conditions persisting on the scale ofminutes are of most relevance to current demands, whereas speed and loadgradients tend to last for only a few seconds.

The applicable measure of demand depends on the criteria in question.For example, if the criteria relates to the amount of NO_(x) stored bythe LNT 105 or the remaining storage capacity of the LNT 105, theexhaust flow rate, the exhaust temperature, and the engine NO_(x)production rate are all relevant measures of demand. If the criteriarelates to efficiency of the LNT 105 as determined using a sensor 110,of the foregoing measures only the engine NO_(x) production rate isrelevant as exhaust flow rate and exhaust temperature directly affectthe LNT efficiency measurement. By contrast, NO_(x) emission ratesincrease with NO_(x) production rates even as LNT efficiency remainsconstant. A greater LNT efficiency may be required to meet emissionlimits when the engine 101 is producing more g/bhp-hr NO_(x) or a higherconcentration of NO_(x), the requirements depending on the emissionlimits. As the demands placed by the engine 101 on the aftertreatmentsystem 103 increase, the threshold for regeneration is lowered andvis-a-versa.

The measure of demand can be used to modify the critical value in anysuitable fashion. For example, the critical value can be set to one ofseveral possible values depending on the measure. As a more specificexample, a loading threshold at which denitration is initiated can beset to a lower value if the engine 101 is in a high speed-high loadcondition as compared to the value used at other times. As anotherexample, the critical value can be made a continuous function of themeasure of demand. Specific examples include linearly decreasing aNO_(x) loading threshold at which regeneration begins as exhaust flowrate increases and linearly increasing an LNT efficiency threshold awhich regeneration begins as engine NO_(x) emission rate on a g/bhp-hrbasis increases.

FIG. 5 is a flow chart of an exemplary control method 500 in which aregeneration threshold is set based on current demands placed by theengine 101 on the exhaust aftertreatment system 103. The method beginsin block 501 during a lean phase. In block 501, a regeneration criteriais set based on current conditions. For example, demands can be relatedto the speed-load state of the engine 101 and using a map from thespeed-load state, a saturation threshold at which regeneration beginscan be set In block 502, a determination is made whether the criteria ismet; in this case whether the saturation exceeds the threshold. If thecriteria is met, the method proceeds to block 503, wherein regenerationtakes place. If the criteria is not met, the method returns to block501, wherein the criteria is reset for the next iteration based on thethen current demand level and subsequently tested in block 502 againstthe updated condition of the exhaust aftertreatment system 103.

A related concept is to vary the endpoint for regeneration based oncurrent demands, When demands placed on the exhaust aftertreatmentsystem 103 are high, it may be necessary to regenerate extensively tomaintain sufficient emission control performance without regeneratingoverly frequently. Likewise, when demand are comparatively low, it maybe sufficient to regenerate to a much lesser extent, and it may bedesirable to regenerate to a lesser extent in order to take advantage ofthe greater efficiency when regenerating at higher loadings. Thisconcept can be implemented by setting a threshold, such as saturationlevel, for ending regeneration based on the current demand state.

The concepts of varying starting and ending points for denitration basedon current demands is also applicable to desulfation. It can bedesirable to postpone desulfation and terminate desulfation early forseveral reasons. One reason is that ammonia production has been observedto increase with sulfur loading. Another reason is that more sulfur canbe removed per unit time at higher sulfur loadings; if sulfur is removedat high loadings rather than low loadings, total desulfation time isreduced. Reducing desulfation time is valuable in that LNTs generallyage and undergo deactivation in proportion to the time they remain atdesulfation temperatures. Reducing desulfation time without increasingdesulfation temperatures extends LNT life.

Combined Method

FIG. 6 is a flow chart of a denitration control process 600 combingseveral of the inventors' concepts. The method starts in block 601,which is the beginning of lean operation during which the LNT 105 isaccumulating NO_(x). The method proceeds to block 602, which checkswhether the saturation of the LNT 105 has exceeded a critical level. Thesaturation of the LNT 105 is the amount of NO_(x) stored in the LNT 105normalized by a theoretical maximum. If it the saturation has exceeded acritical amount, the method proceeds to block 603, which checks whetherthe NO_(x) mitigation performance of the LNT 105 has fallen below acritical value. If the NO_(x) mitigation performance of the LNT 105 hasfallen below a critical value, the method proceeds to block 604, whichinitiates rich regeneration. If either the test of block 602 or 603 isfailed, both tests are repeated at the next iteration. If the test ofblock 603 fails, the test of block 602 is repeated even though it waspreviously passed because the criteria applied for the next iterationmay be different and may demand a higher level of saturation prior toallowing regeneration.

After regeneration begins in block 604, the process 600 proceeds toblock 605, which checks whether a criteria relating to the end ofdenitration has been met. In this example, the criteria relates to aNO_(x) saturation of the LNT 106. A saturation estimate is maintainedthroughout the process 600, whereby the estimate increases as the leanphase progresses and decreases as the rich phase progresses. Theincrease and decrease rates are estimated with models. The regenerationprocess continues until the saturation is reduced to the target level,whereupon a new lean phase is begun in block 601.

Several processes occur in parallel to the core process defined byblocks 601-605. The process defined by blocks 610 and 611 updates thecritical value against which the LNT saturation is tested in block 602.Block 610 sets a maximum saturation that the LNT 106 can be allowed toreach. This saturation is based on the requirements of the currentoperating state of the power generation system 100. The speed-load pointof the engine 101 is a primary factor determining those requirements. Inone example, the maximum saturation is determined based on a map fromthe current speed-load point. A map can take the form of a table or afunctional formula, for example.

Block 611 lowers the critical value based on the conduciveness ofcurrent conditions to regeneration. In general, the more conducive thecurrent conditions are, the more the critical value is lowered. Aprinciple factor in determining conduciveness is the exhaust state,which is primarily a function of the engine speed-load history.Accordingly, the reduction in critical saturation applied in block 611can be determine based on another map from the current speed-load point,optionally modified based on an engine speed or load gradient.

Blocks 610 and 611 combine to determine one critical value. Theconsiderations applied by blocks 610 and 611 are different and theparameters affecting the outcomes of these block can be different, butthe two blocks together lead to one result. The process of block 610 and611 can be combined by defining one map that directly relates theparameters used by the two blocks collectively to the final decision asto the critical value that will be applied in block 602. Thus the targetsaturation can be set based on both the demand for NO_(x) mitigationunder current condition and conduciveness of current conditions toregeneration in one step.

Another parallel process is defined by blocks 612 and 613. The blocksare analogous to those of block 610 and 611, except that instead ofdetermining a critical saturation, they determine a critical NO_(x)mitigation performance of the LNT 105 or of the LNT 105 in combinationwith the SCR catalyst 106. These two operations can also be combinedinto one. Another parallel process is defined by blocks 614 and 615,which determine the endpoint for denitration. The description of theseblocks parallels that of blocks 610 and 611.

Additional sub-processes are defined by blocks 620-623, which relate tothe determinations of LNT performance and LNT saturation used in thecomparisons of blocks 602 and 603. At the completion of denitration inblock 605, a signal is passed to block 620 which begins a program ofNO_(x) sensor data weighting for a post regeneration phase. Block 620produces weighting factors which change over time and are used by block621. Block 621 estimates a NO_(x) concentration downstream from the LNT106. Block 621 uses information from a NO_(x) sensor and from a model ofthe system 100. The model can use various information, such as thespeed-load point of the engine 101. Block 621 can be, for example, aKalman filter. The weighting factors supplied by block 620 cause block621 to give less weight to the NO_(x) sensor data in a periodimmediately following the completion of a denitration. The NO_(x)concentration estimate is used to estimate NO_(x) mitigation efficiencyin block 622 and to update the estimate of LNT saturation in block 623.

Setting Regeneration Threshold Based on Engine Speed Load Information

An alternate way of viewing some of the inventors' concepts is in termsof the types of parameters that are considered in setting a thresholdfor initiating denitration. As shown by the foregoing, at least two ofthe inventors' concepts involve consideration of speed-load informationfrom the engine 101 and using that information together with anindependent determination of the state and or performance of the exhaustaftertreatment system 103 in determining the time at which to beginregeneration. Accordingly, another of the inventors concepts is a methodof scheduling denitration wherein a threshold for denitration is variedbased on engine speed-load information. The threshold is a criticalvalue for a measure of state and/or performance of the exhaustaftertreatment system 103 or a portion thereof comprising an LNT 105.

The threshold can be set in view of multiple consideration with theultimate objective generally being to minimize the fuel penalty foroperating the exhaust aftertreatment system. As noted above, someoperating conditions allow regeneration to be postponed due to lowerdemands for exhaust aftertreatment but where regeneration can bepostponed, it is still sometimes desirable to advance regenerationtiming to take advantage of opportune conditions and to avoid adverseconditions that may subsequently occur. Taking all the considerationtogether, there is an optimal threshold that can be associated with anengine speed-load point or an engine speed-load point in combinationwith historical engine speed-load information, such informationdetermining the engine speed-load gradient.

Any suitable approach can be used to determine the map from enginespeed-load information to the threshold. In one embodiment, a set ofdata provides training examples used determine the map. Data fromvehicle operating cycles is valuable in that it captures informationgoing beyond current operating conditions. For example, in terms ofopportunistic regeneration, it is best to consider more than just thedegree to which current condition are conducive to regeneration indeciding how much to alter the threshold and advance regenerationtiming. In addition to their conduciveness, the likelihood of currentconditions to persist is relevant. When persistence is more likely,there is less advantage to advancing the regeneration timing than ifpersistence is less likely. Thus, in building the map, it is desirableto process the training examples to capture any correlation between thespeed-load point and gradient and the persistence of conditions at thatspeed-load point and gradient. Such correlations can be captured byprocessing the training examples in a suitable manner. One option is tobuild a probabilistic dependency model that can be used to calculateoptimal thresholds to use for regeneration as a function of enginespeed-load point or speed load point in combination with speed-loadgradient. In general, even better performance can be obtained byconsidering additional parameters to engine speed-load information insetting the thresholds. Additional parameters could relate, for example,to the exhaust system state, e.g., LNT temperature, to the vehiclespeed, or to the engine speed-load history.

The invention as delineated by the following claims has been shownand/or described in terms of certain concepts, components, and features.While a particular component or feature may have been disclosed hereinwith respect to only one of several concepts or examples or in bothbroad and narrow terms, the components or features in their broad ornarrow conceptions may be combined with one or more other components orfeatures in their broad or narrow conceptions wherein such a combinationwould be recognized as logical by one of ordinary skill in the art.Also, this one specification may describe more than one invention andthe following claims do not necessarily encompass every concept, aspect,embodiment, or example described herein.

1. A method of operating a power generation system, comprising:operating a diesel engine to produce a lean exhaust comprising NO_(x);passing the lean exhaust through an exhaust aftertreatment systemcomprising a lean NO_(x) trap to adsorb a portion of the NO_(x) from theexhaust; and selectively denitrating the lean NO_(x) trap based on theNO_(x) loading of the lean NO_(x) trap reaching a critical amountwherein the critical amount is varied according to the exhaust flowrate; wherein denitrating the lean NO_(x) trap comprises providing anoverall rich exhaust-reductant mixture to the lean NO_(x) trap, wherebythe lean NO_(x) trap releases and reduces stored NO_(x); and thecritical amount is varied so as to lower the NO_(x) saturation at whichdenitration takes place when the exhaust flow rate increases.
 2. Amethod of operating a power generation system, comprising: operating adiesel engine to produce a lean exhaust comprising NO_(x) passing thelean exhaust through an exhaust aftertreatment system comprising a leanNO_(x) trap to adsorb a portion of the NO_(x) from the exhaust;selectively denitrating the lean NO_(x) trap based on the NO_(x) loadingof the lean NO_(x) trap reaching a threshold; and setting the loadingthreshold based on operating conditions to advance regerneration whenthe operating conditions place high performance demands on the exhaustaftertreatment system, independently of the way those conditions affectthe state of the exhaust aftertreatment system, and to postponeregeneration when the operating conditions place comparatively lowerdemands on the exhaust aftertreatment system; wherein the state of theexhaust aftertreatment system consists of the NO_(x) loading, the NO_(x)storage capacity, and the temperature of the lean NO_(x) trap;denitrating the lean NO_(x) trap comprises providing an overall richexhaust-reductant mixture to whereby the lean NO_(x) trap, whereby thelean NO_(x) trap releases and reduces stored NO_(x); and the thresholdis set according to one or more of the exhaust oxygen flow rate, theexhaust oxygen concentration, and the exhaust flow rate.